The electrochemical behavior of electrodes made by sealing carbon nanofibers in glass or with electrophoretic paint has been studied by scanning electrochemical microscopy (SECM). Because of their small electroactive surface area, conical geometry with a low aspect ratio and high overpotential for proton and oxygen reduction, carbon nanofiber (CNF) electrodes are promising candidates for producing electrode nanogaps, imaging with high spatial resolution and for the electrodeposition of single metal nanoparticles (e.g., Pt, Pd) for studies as electrocatalysts. By using the feedback mode of the SECM, a CNF tip can produce a gap that is smaller than 20 nm from a platinum disk. Similarly, the SECM used in a tip-collection substrate-generation mode, which subsequently shows a feedback interaction at short distances, makes it possible to detect a single CNF by another CNF and then to form a nanometer gap between the two electrodes. This approach was used to image vertically aligned CNF arrays. This method is useful in the detection in a homogeneous solution of short-lifetime intermediates, which can be electrochemically generated at one electrode and collected at the second at distances that are equivalent to a nanosecond time scale. A nanogap is an electrode arrangement in which two collinear electrodes are separated by a gap of nanometer dimensions. There have been a number of studies involving nanogaps, most of these involving studies of the electronic transport properties of single molecules, which bridge the gap (e.g., DNA or other macromolecules). 1,2 Nanogaps are usually prepared with fixed dimensions, e.g., by mechanically produced break junctions, 3 break junctions formed by electromigration, 4 electrodeposition, 5 carbon nanotube extracted lithography, 6 direct e-beam lithography, 7 and conventional microscale fabrication techniques such as optical lithography, electron-beam evaporation, and liftoff. 8 In most cases, the exact gap dimensions are uncontrolled, although there have been reports of the formation of controlled nanogaps of fixed dimensions by several different approaches. 9-11 Our group has been interested in nanogaps whose dimensions are continuously variable, for example, in connection with studies of single-molecule electrochemistry 12 and in studies of rapid homogeneous reactions coupled to electron-transfer reactions at electrodes. 13 Generally, kinetic studies of electrochemically generated, unstable species can be achieved if the species lifetime is of the order of the diffusion time across the gap, d 2 /2D, where d is the gap separation and D is the diffusion coefficient. To increase the range of systems that can be studied by scanning electrochemical microscopy (SECM) to chemical species with lifetimes in the microsecond and nanosecond region, one must develop experimental strategies for decreasing d to the nanometer level, i.e., to a nanogap. This can be achieved by using extremely small probes (UMEs) that can approach a substrate or another electrode to very small distances. With glass-insulated disk electrodes, of the type most frequently used in SECM, one is limited by the RG value of the electrode, i.e., the ratio of the radius of glass insulating shroud to that of the metallic disk electrode. Another problem associated with disk UMEs is that the exposed electroactive part is sometimes recessed slightly beneath the insulating layer. While this has only a minor effect during micorometer-scale measurements, any feedback-based approach to a nanometer distance often results in the insulator touching the substrate and blocking any further movement of the recessed electroactive disk. A useful approach to solving this problem is to use conical electrodes that can approach the substrate or another similar electrode to very small d values without colliding. Conical UMEs have been previously studied by a combination of scanning electron microscopy (SEM), steady-state voltammetry
